The aircraft floor at the level of the center wing box is subjected to many stresses due to the distortion of the center wing box, particularly in flight. FIG. 1 shows a cross section of an aircraft fuselage 1 at the level of the center wing box while the aircraft is in flight. Compression and tension forces applied to the wings 2 tend to extend the lower surface 3 of the wings 2 and conversely, to shorten the upper surface 4 of the wings 2. Such a distortion of the wings 2 causes a similar distortion of the center wing box 5 and of the floor structure 6 above said wing box 5. In the case where the floor structure is rigidly tied to the upper surface 4 of the wings 2, these distortions can lead to cracks or to a weakening of the floor structure 6.
FIGS. 2 and 3 of the state of the art show a cross section of an aircraft floor structure at the level of the center wing box.
A floor structure 6 consists of a series of longitudinal beams 8 intended to strengthen the lower floor panel 7 of the fuselage section and is aligned with the upper surface 4 of the wings. Said longitudinal beams 8 are fastened to the lower or upper face of the said floor panel, the upper face being in the internal volume of the fuselage and the lower face being in the internal volume of the center wing box 5. The longitudinal beams 8 are also attached to a number of vertical connecting rods 13 placed between the longitudinal beams and the bottom panel 12 of the center wing box 5.
In addition, several longitudinal rails 9 are laid above the longitudinal beams 8. In that way, the several longitudinal beams 8 support the entire length one or several longitudinal rails 9, which are tied to them by vertical rods 10.
In some aircraft and in particular those that are designed for freight, the floor structure 6 includes also longitudinal rails 11 directly fastened to the lower floor panel 7 of the fuselage by vertical rods 10, and are therefore independent of strengthening longitudinal beams.
Thus on FIGS. 2 and 3 of the state of the art, the floor structure includes two central longitudinal rails 11, two left longitudinal rails 11 and two right longitudinal rails 11 which are independent from the longitudinal beams.
Yet, whereas the longitudinal rails 9 attached to the longitudinal beams 8 are able to follow the distortions of the lower floor panel 7 of the fuselage, the central and side longitudinal rails 11, directly fastened to the lower floor panel 7 of the fuselage by means of vertical rods 10, are rigidly tied to the said panel 7 and can not follow the floor distortions. Such a rigid link may lead to excessive stress causing cracks in the floor structure 6 at the center wing box 5 level.
The invention seeks to provide an aircraft floor, notably intended to be the floor of a fuselage section containing the center wing box, which can adapt to the distortions caused by the wings movement. To that effect, the invention proposes to disassociate the distortions of the general structure of the fuselage section and that of the floor itself, in order to lower the forces on the said floor. More precisely, the invention proposes to disassociate the work of the floor and that of the support surface on which the floor is attached.
In the fuselage section according to the invention, the floor is fitted with longitudinal rails linked to the floor support surface by means of longitudinal beams fixed on said support surface. Longitudinal in this description means parallel to the longitudinal axis of the aircraft. The support surface consists of a lower panel of the particular fuselage section, which may be in the case of the center section, the upper panel of the center wing box. The longitudinal rails, which in state of the art aircraft are not fixed on the longitudinal beams and are thus directly linked to the support surface, are henceforth attached to at least one transversal crosspiece that can expand or retract in order to follow the lengthening or shortening of the support surface. Transversal in this description means perpendicular to the longitudinal axis of the aircraft. The transversal crosspiece is linked to the support surface by means of swivel linkages allowing the crosspiece to follow the distortions of the support surface. For example, the crosspiece features two pivoting elements, the second being able to swivel relative to the first in order to transversally lengthen or shorten the largest dimension of said crosspiece. The crosspiece according to the invention is not subject to wing loads, but supports only vertical forces.
Therefore, the object of the invention is an aircraft fuselage section comprising several longitudinal strengthening beams attached to a lower panel of the fuselage and several longitudinal floor support rails, characterized by at least one crosspiece extending transversally between two strengthening beams and supporting a longitudinal rail, said crosspiece being able to swivel on the lower panel of the fuselage in such a way as to permit its rotation around a longitudinal axis.
The opposite ends of the crosspiece are each fixed to one of the two longitudinal beams between which the crosspiece is mounted.
Swivel in this description means that the crosspiece can pivot from one position to another by rotating around an axis parallel to the fuselage section longitudinal axis. Such a rotation of the crosspiece allows it to follow the transversal distortion of the lower panel of the fuselage section.
The crosspiece can be mounted to swivel directly on the fuselage section lower panel, or attached to one or both of the strengthening beams between which it is installed, these being fixed on the aircraft lower panel.
Some embodiments of the fuselage section according to the invention allow for all or part of the following characteristics:                The fuselage section, and more specifically the floor structure of said section, comprising several crosspieces extending successively between at least two strengthening beams, at least one longitudinal rail extending longitudinally between two strengthening beams and supported by a series of crosspieces. The longitudinal rail is thus supported along its entire length by crosspieces bound to the fuselage section lower panel. The crosspieces are placed one behind the other with respect to the longitudinal axis of the aircraft section.        at least one crosspiece features a double pivot articulation, the first pivot being attached to the main body of the crosspiece and the second pivot being attached to the fuselage section lower panel.        
The presence of this articulation that can pivot from a position in line with the main body to a position perpendicular to the main body for example permit the lengthening or shortening of the crosspiece ensemble in order to follow the lengthening or shortening of the fuselage section lower panel. Then, the longitudinal rail which is supported by the crosspiece is no longer subjected to the distortions of the fuselage section lower panel.
Advantageously, the main body is fitted at one first end with the double pivot articulation while the other end is fitted with a pivot linkage.
In some fuselage sections, and particularly at the level of the center wing box, the floor structure is vertically separated from the fuselage section lower panel. In other words, there is a certain height between the fuselage panel and the floor structure. Height in this description means the vertical distance from the floor structure. It is also necessary to use vertical linkages, such as vertical rods, to fasten the floor structure to the fuselage lower panel. In the invention, according to needs, at least one longitudinal rail is designed to be attached to a crosspiece by means of a vertical rod. The vertical rod can then be mounted swiveling on the crosspiece around a transversal axis.
Preferably, at least one longitudinal rail is fixed to a strengthening beam along which it is laid.